How to enroll in the Office Insider program on Windows 10

Here's how to quickly become an Office Insider on your PC and Mobile to get early access to features and improvements and how to opt out if things aren't working as expected.
Similar to Windows 10, Office also has an Insider program that allows any Office 365 subscriber to get early access to features and improvements, which eventually will be part of the most popular suite of apps in the world.
Office Insider is available on two levels: "Insider Slow" or "Insider Fast." In the Insider Slow, new versions of the suite rollout at a slower pace, at least once or twice a month, with new features without significant risks of bugs or things not working correctly.